Subsea centrifugal compressor with horizontal shaft and with only one axial thrust bearing

ABSTRACT

A centrifugal compressor unit including a motor having a rotor, at least one compressor having a shaft driven by the rotor; and a set of bladed wheels fitted on the driven shaft, the motor and the compressor being mounted in a common watertight casing. It further includes cooling means for cooling the motor, the cooling means having a gas cooler outside the casing. In operation, the motor, the compressor and the casing having the motor and the compressor are arranged horizontally, one axial thrust bearing only being provided on a compressor side.

BACKGROUND OF THE INVENTION

Embodiments of the invention relate to a turbocompressor or a motorcompressor, and particularly to an integrated motor compressor unit.

More particularly, embodiments of the present invention relate to amotor compressor unit for subsea applications.

Motor compressor units of the integrated type comprise a motor means,generally an electric drive motor, and a compressor having one or morecompression stages, depending on the applications, the motor means andthe compressor being mounted in a common casing that is sealed againstthe gas handled by the compressor unit.

Each compression stage comprises a bladed impeller mounted on a shaftdriven by a rotor, which is driven by the motor means.

According to the state of the art, when used under special operatingconditions, especially for underwater operations, the motor compressoris usually arranged vertically to facilitate draining of the interior ofthe casing.

A compressor unit of this type is already known from US 2009/0 220 362in which, in addition, the shaft of the compressor and the rotor drivenby the motor means are coupled to form one common shaft that rotatesaround a vertical axis. The shaft and the rotor are coupled using atwistable shaft that transmits the rotational moments between the motormeans and the compressor and, as far as possible, mutually decouples thedriven shaft and the rotor from vibrations.

Besides, in operation, the pressure difference on the opposite sides ofthe compression stages generate a thrust on the compressor that istransmitted via the twistable coupling to the rotor and to a thrustbearing provided at one end of the common shaft, namely on the upper endof the of the motor rotor. The common shaft being arranged vertically,only one common thrust bearing is required for the motor and thecompressor.

The thrust is directed in opposition to the weight forces of the commonshaft, including the twistable coupling and the compression stages, suchthat a thrust balance is in addition carried out by the weight of thecommon shaft.

However, this type of arrangement in which the rotor is axiallysupported by one thrust bearing has the disadvantage that the couplingbetween the rotor of the motor and the compressor shaft may beprejudicial to the accuracy of the axial position of the rotatingelements of the compressor which may impact the motor compressorefficiency, in particular during transient phases.

This problem may in particular incur at high loads, due to a thermalexpansion of the compressor shaft driven by the motor means and mayarise for compressor units having a high pressure ratio, namely above 3,and having an increased number of compression stages, equal or higherthan 3.

Other arrangements of motor compressor units are known, having a commoncasing in which are mounted a compressor having one or severalcompression stages mounted on a compressor shaft and a motor having arotor driving the shaft of the compressor around a vertical axis, and inwhich the compressor shaft and the rotor are assembled together using arigid coupling to form a common shaft supported by three radialbearings.

Such an arrangement does not permit, however, to increase the number ofthe compression stages such that the motor compressor unit cannot beequipped with more than three compression stages.

SUMMARY OF INVENTION

In view of the foregoing, one object of embodiments of the presentinvention is to alleviate the above-mentioned drawbacks of the motorcompressor units according to the state of the art.

A further purpose of embodiments of the present invention is to providea centrifugal compressor unit for subsea applications in which theproblem of axial displacement of the bladed wheels mounted on thecompressor shaft are overcome.

Embodiments of the invention thus propose a subsea centrifugalcompressor unit, comprising:

a motor having a rotor,

at least one compressor having a shaft driven by the rotor, and

a set of bladed wheels fitted on the driven shaft.

According to a general feature, in operation, the motor, the compressorand the casing having the motor and the compressor are arrangedhorizontally, one axial thrust bearing only being in addition providedon a compressor side.

According to another feature of embodiments of the invention, the rotorand the driven shaft form one common shaft, said compressor unit havinga set of three radial bearings for supporting said shaft, said set ofradial bearings comprising two bearings being provided on the motor sideand one bearing being provided on the compressor side.

According to another embodiment, the rotor and the driven shaft areconnected by means of a flexible coupling that is tortionally flexibleand axially unflexible, the compressor unit further comprising a set offour radial bearings for supporting each end of the rotor and the drivenshaft.

For example, the bearings, namely the axial bearing and the radialbearings for supporting the driven shaft or the rotor are magneticbearings.

According to a further feature, the gas cooler is part of a closed loopextending partly outside the casing.

Besides, a seal may be provided between the motor, downstream thecooling means, and the compressor.

In an embodiment, the casing is pressurized.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the invention will becomeapparent from the following description, given only by way of exampleand in view of the following drawings in which:

FIG. 1 illustrates the general features of a centrifugal compressorunit; and

FIG. 2 illustrates a further aspect of a centrifugal compressor unit.

DETAILED DESCRIPTION

FIG. 1 shows a centrifugal compressor unit 1 according to embodiments ofthe invention, for subsea applications, having a motor 2 and one or morecompressor(s) 3 mounted in a common casing 4 leak tight to the gasmanipulated by the compressor 3.

The casing 4 comprises two casing elements fixed together by appropriateattachment means, the one, denoted by numeral reference 5, accommodatingthe motor 2 and the other, denoted by reference 6, accommodating thecompressor.

The casing elements are intended to provide support and protection forthe motor and the compressor, respectively. As illustrated, the casing 2is provided with a gas inlet orifice I facing the first compressionstage of the compressor and an outlet orifice O facing the lastcompression stage.

In operation, the casing element assembly is intended to be pressurizedand immersed and is arranged on a base such that the compressor unit isarranged horizontally.

In the exemplary embodiment, the motor 2 is formed by an electric motor,for example a high rotation speed motor (6000 to 16 000 rpm) supplied bya frequency variator and comprising a stator 7 and a rotor 8 supportedby two radial end radial bearings 9 and 10.

As illustrated, the compressor 3 comprises a shaft 11 driven by themotor rotor and supported by a set of two radial end bearings 12 and 13and a set of bladed wheels 14 mounted on the driven shaft and definingeach a compression stage for the compressor.

Therefore, the rotor of the motor turns in two radial end bearings 9 and10, whereas the compressor shaft turns in two radial end bearings 12 and13.

In an embodiment, the bearings are rigidly fixed to the casing and areformed by active magnetic bearings that are controlled such that theycan adapt to the dynamic behavior of the rotor or the driven shaft thatthey support.

As illustrated, the rotor 8 and the driven shaft 11 are linked via aflexible coupling that is radially flexible and axially unflexible todynamically reduce the amplitude of the noise generated by ventilationwhen the compressor is rotated while efficiently propagating an axialthrust generated by the bladed wheels to the rotor of the motor.

As a matter of fact, in operation, the pressure differences generated ateach compression stage generate a thrust on the driven shaft which istransmitted to an axial thrust bearing 16, for example an axial activemagnetic bearing that is provided at the free end of the driven shaft11, namely at the end opposite to the rotor of the motor 2.

As illustrated, in an embodiment, the axial bearing 16 is located on thecompressor side, namely as close as possible of the active portion ofthe compressor shaft such that the axial bearing 16 is more appropriateto compensate for the thermal expansion of the shaft and to moreaccurately guarantee the axial position of the driven shaft and of therotating bladed wheels within the second casing element 6.

It should be appreciated that thanks to the use of the flexible couplingthat is tortionally flexible and axially unflexible, each of the rotorand the driven shaft can keep it on vibration behavior while the thrustgenerated during operation of the completion stages is supported by onefirst bearing only provided on the compressor side of the compressorunit.

In addition, with such an arrangement in which the rotor and the shaftare connected by a flexible coupling and having four radial bearings andone axial bearing, the compressor unit may comprise a large number ofcompression stages, namely above 3, and provide a high pressure ratio,namely above 3.

It has been noted that such a subsea compressor unit may provide apressure between 80 and 150 bars.

For example, the compressor may comprise up to 8 compression stages andprovide a pressure ratio up to 3 or 3.5.

Referring now to FIG. 2, in one embodiment, the subsea centrifugalcompressor unit may be provided with cooling means for cooling the motoras well as the bearings.

In one embodiment, the subsea compressor unit may be provided with acooling system having a cooling loop lying between the first compressionstage and one side of the motor opposite to the compressor or lyingbetween a both sides of the rotor, as illustrated, to provide cooling inclosed loop.

In such a case, the cooling system comprises a gas cooler 18 lyingoutside the casing 2 and therefore in a contact with the cold deep waterat constant temperature.

According to another embodiment, the cooling means may be arranged as anopen loop with a clean gas supply 19 entering the first casing element 2at one side of the rotor opposite to the compressor.

In such a case, the rotor may be provided with an additional bladedwheel 20 such that the clean gas supply is directed to the motor,through the radial bearing 9 and to the compressor through a barrierseal 21 provided between the flexible coupling 15 and the compressor.

Referring back to FIG. 1, an additional axial thrust bearing 17 may beused, in such a case, to compensate for the thrust generated by themotor internal pressure drop to the cooling flow.

At last, it should be noted that the invention is not limited to thedisclosed embodiment.

As a matter of fact, according to the exemplary embodiments disclosedwith reference to FIGS. 1 and 2, the compressor unit comprises a rotorand a shaft supporting the compression stages that are linked using aflexible coupling and having four radial bearings for supporting therespective free ends of the rotor and of the driven shaft.

According to another embodiment, the compressor unit may comprise acommon rigid shaft or a rotor and a stator that are linked using a rigidcoupling, namely a coupling that is axially and tortionally unflexible.

With such an arrangement, the common shaft may be supported by tworadial bearings on the motor side and by one radial bearing at thecompressor side, with one axial bearing only. This arrangement alsopermits to have a large number of compression stages, namely above 3,and provide a high pressure ratio, for example above 3.

This written description uses examples to disclose the invention,including the preferred embodiments, and also to enable any personskilled in the art to practice the invention, including making and usingany devices or systems and performing any incorporated methods. Thepatentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those skilled in the art. Suchother examples are intended to be within the scope of the claims if theyhave structural elements that do not differ from the literal language ofthe claims, or if they include equivalent structural elements withinsubstantial differences from the literal languages of the claims.

What is claimed is:
 1. A subsea centrifugal compressor unit, comprising:a motor having a rotor, at least one compressor having a shaft driven bythe rotor; and a set of bladed wheels fitted on the driven shaft, themotor and the compressor being mounted in a common watertight casing,and further comprising cooling means for cooling the motor, the coolingmeans having a gas cooler outside the casing, wherein, in operation, themotor, the compressor and the casing having the motor and the compressorare arranged horizontally, and in that it further comprises one axialthrust bearing only on a compressor side.
 2. The subsea centrifugalcompressor unit according to claim 1, wherein the rotor and the drivenshaft form one common shaft, the compressor unit having a set of threeradial bearings for supporting the shaft, two bearings being provided onthe motor side and one bearing being provided on the compressor side. 3.The subsea centrifugal compressor unit according to claim 1, wherein therotor and the driven shaft are connected by means of a flexible couplingthat is tortionally flexible and axially unflexible, and furthercomprising a set of four radial bearings for supporting each end of therotor and the driven shaft.
 4. The subsea centrifugal compressor unitaccording to claim 2, wherein the radial bearings for supporting thedriven shaft or the rotor are magnetic bearings.
 5. The subseacentrifugal compressor unit according to claim 1, wherein the axialbearing is a magnetic bearing.
 6. The subsea centrifugal compressor unitaccording to claim 1, wherein the gas cooler is part of a closed loopextending partly outside the casing.
 7. The subsea centrifugalcompressor unit according to claim 1, comprising a seal between themotor, downstream the cooling means, and the compressor.
 8. The subseacentrifugal compressor unit according to claim 1, wherein the casing ispressurized.